Currently only literal `0` and `1` are accepted as `read`/`write`
flags.
This patch allows other boolean arguments, C macros (`FALSE`/`TRUE`),
Ruby `VALUE`s (`Qfalse`/`Qtrue`), and C99 `bool`s (`false`/`true`), as
well.
https://github.com/ruby/rdoc/commit/169dc02e3c